Telephone and other electric wave transmission systems



12, 1941- R. J. HALSEY 2,252,002

TELEPHONE AND OTHER ELECTRIC WAVE TRANSMISSION SYSTEMS Filed Jan. 11, 1940 /C/6./. F/@. 2 W /6 2/23 24 Z 27 H? INVENTOR REGINALD JOHN HALSEY BY 6 22 ATTORNEY;

Patented Aug. 12, 1941 TELEPHONE AND OTHER, ELECTRIC WAVE TRANSMISSION SYSTEMS Reginald John Halsey, Radlett, England Application January 11, 1940, Serial No. 313,353 In Great Britain January 24, 1939 5 Claims.

This invention relates to electric wave systems of the type in which means are incorporated for the purpose of varying the amplification or attenuation at some point as a function of signal amplitude. More particularly the invention is concerned with electric communication systems in which devices of this type are incorporated at two points so that at one point the ratio of maximum to minimum amplitude signals is decreased, whilst at a second point the ratio of signal amplitudes is increased to the original value. The means by which such operation is obtained may be referred to as compressor and expander respectively and the complete equipment has sometimes been termed a compander. The invention is not limited to electric communication systems, and may be used in any circumstances where it is desired to limit amplitude-or amplitude range at some point, for example in gramophone and other sound recording,

In a complete system, in order to ensure faithful reproduction of the applied signal the expander must follow a law inverse to that of the compressor. On twd-way communication circuits without singing suppressors this is also very desirable in order that stability shall be ensured at all signal levels.

The most direct method of effecting this operation is to provide a variable attenuation device at the compressor, and to make the corresponding attenuation device at the expander obey a law which is the inverse of the first. Such characteristics, however, are diflicult to attain, particularly where non-linear characteristics are concerned.

It is much easier to make use of variable attenuation (or variable impedance) devices which are identical or similar in characteristic and to arrange that the amplification or attenuation at one point is made inversely proportional to the characteristic of the device there incorporated. By similar characteristics is to be understood that the characteristics obey similar laws of variation, corresponding impedances being related by a constant factor which need not be unity.

One such system of this type which has been proposed has involved connecting a replica of the compressor unit in the negative feed back path of an amplifier in the expander unit, but this system has certain disadvantages, notably in that in order that the attenuation may be controlled proportionally with change of characteristic of the feed back path the amplifier has to be of high forward gain with a considerable degree of degeneration.

Another, and much simpler, system involves the use of variable attenuation devices in series with and in shunt to the transmission path at the compressor and expander respectively and it is tothis form of apparatus that the present invention relates. The principal object of the present invention is to provide a compander system and parts thereof which are improved in certain directions in order to provide appara tus which is suitable for use in the stringent conditions of telephone communication, though the apparatus improved by the present invention is improved in its operation in other apparatus.

According to the present invention there is provided an electric wave transmission device comprising an input circuit and an output circuit and a tranmission path extending between said circuits, a variable impedance device in" cluded in series in said transmission path, a finite and constant impedance in shunt to said transmission path,,and means for controlling the impedance of said variable impedance as a function of a signal applied to said input circuit.

A further feature of the invention consists of an electric wave transmission device comprising an input circuit, an output circuit and a transmission path extending between said circuits, a variable impedance device having, in the absence of a signal in said input circuit, a high impedance, and a decreasing impedance with increasing signal in said circuit, and a finite constant impedance in shunt to said output circuit.

The invention also comprises an electric Wave transmission device comprising an input circuit, and an output circuit and a transmission path extending between said circuits, a variable impedance device included in series in said transmission path, a finite and constant impedance in shunt to said transmission path, and means for controlling the impedance of said Variable impedance as a function of a signal applied to said input circuit, said device presenting substantially constant and predetermined impedance when viewed from said input and from said output circuit throughout the range of amplitude of signals applied to said input circuit.

The invention still further comprises an electric wave transmission system including a device comprising an input circuit of impedance Z1, a variable impedance device connected, in series with a constant impedance Z2, in shunt to said circuit, a further device connected to said first device and having input circuit of impedance Z3 an output circuit of impedance Z4 and a variable impedance device of the same law of variation as said first variable impedance arranged, with an impedance Z5 in parallel therewith, in series in the transmission path through said second device, and means for controlling said variable impedances in accordance with the amplitude of signals passing through the system,

wherein the said impedances are substantially in accordance with the relations:

In the accompanying drawing are shown by way of example embodiments of the invention.

Figures 1 and 2 are equivalent diagrams of a compressor and expander, respectively, according to the invention; Figs. 3 and l are circuit diagrams of the compressor and expander, respectively; and Figs. 5 and 6 are circuit diagrams of a modified compressor and expander, respectively.

Referring now particularly to Figure 1, a voltage generator having an electromotive force E1 and an impedance Z1 is shown as supplying a circuit comprising a constant impedance Z2 in series with a variable impedance fie) which is a function of an applied control voltage. This control voltage is dependent upon the magnitude of the output voltage 6 which, for example, may be applied to a high impedance grid circuit of a valve amplifier, from the output of which is derived both the control voltage and the transmitted signal. In Figure 2, a voltage generator having an impedance Z3 has an electromotive force 2, which is proportional to, and derived from the output voltage of the system shown in Figure 1;

If the expander is to correct exactly for the distortion produced by the compressor, E2 must be independent of fie). There are two solutions to this expression but only one allows of any compressor action; this is when both the following expressions are independent of fie):

age) (1) and Z1+ Z2 +f( 5+f( When this is the case,

and

i. e. (Za+Zi) in parallel with Z5.

In designing a circuit to comply with these conditions it may be that certain of the impedance values must be given certain values; for example, in designing a compander system of this type for use on telephone communication systems Z1, Z3 and Z4 may be determined by line impedances.

In such a case, to ascertain the values of Z2 and Z52 Substituting the value of E2 from (3), (4) becomes If Z1 is large compared with (Za-I-Zr), i. e. referred to circuits involving the same value of fie), as will frequently be the case in the application of this invention Thus, for example, if Z1 is large compared with Z2 and fie), and if Z3 is small compared with Z4 and fie) Z2=Z4 and Z5 can be omitted.

It will be observed that, although in the theory developed above, the variable impedances are determined by the voltage actually transmitted by the system, i. e. by the output from the compressor and the input to the expander, a similar result may be obtained if the two non-linear networks are controlled by any common or equal voltages. Thus, if the original signal and the final output signal are equal these may be used for derivation of the control voltages at the compressor and expander respectively,

In the foregoing it is assumed that fie) is reduced as 6 increases. If fie) is increased as 6 increases the so-called compressor becomes an expander and vice versa.

Figures 3 and 4 of the drawing are circuit diagrams of a compressor and expander respectively. In Figure 3, signals applied to input terminals II! are fed, through a resistance H, to the input winding l2 of a transformer of which the secondary winding [3 feeds the winding [4 of a further transformer; the secondary winding 15 of this latter transformer supplies output terminals 13 which are closed by a high impedance such as the grid circuit of a valve amplifier. shunted across the loop circuit between windings I 3, I4, is a variable impedance composed of two rectifiers ll, [8, of the metal, dry contact type, in series with a resistance 59. The impedance of the rectifiers may be controlled by a current supplied to terminals 28 and if this current is made proportional to the output signal, for example by being derived therefrom by rectification, the attenuation produced will vary with the signal anialitude and volume compression will resu Figure 4 indicates the corresponding volume expander. Input terminals 2|, shunted by resistance 22, are connected to winding 23 of a transformer whose secondary winding 24 is connected through rectifiers 25, 26 to a transformer winding 27, and this transformer in turn has a secondary winding 28 feeding output terminals 29 which are closed by a high impedance such as the grid circuit of a valve amplifier. The loop circuit between transformer windings 24, 21 includes shunt resistances 30, 3| and control current is applied to terminals 32.

In designing the circuits of Figures 3 and 4 to operate on a telephone transmission line where the input in each case is to be a 600 ohm line and the output is to be fed to the input of a valve amplifier the following values may be adopted:

Ohms Resistance II 600 Resistance l9 32 Resistance 22 600 Resistance 30 200,000 Resistance 3| 20 Turns ratio of windings l2, l3 1:13 Turns ratio of windings l4, l 1:6 Turns ratio of windings 23, 2t 5:1 Turns ratio of windings 21, 28 1:130

and may be varied in accordance with circuit requirements.

In general the effect of the series resistance l9 associated with the compressor is to limit the volume range over which the compressor is effective. small by comparison with all working values of fie). In practice, therefore. it may be desirable to omit the resistance from the compressor circuit where this may be done without substantial change of the compression law. Further it may be required to limit the volume range over which compression or expansion occurs and this may be done by suitably increasing the value of the appropriate resistances i9 and 3|.

The compression law commonly required from this type of equipment is a 2:1 law, i. e. for each 2 db. reduction of the input voltage, the output voltage is reduced by 1 db. The circuits shown can be adapted to give approximately such a law I over a wide range of input voltages but it will be understood that impedances may be added to the compression and expansion networks shown in order to match this or any other desired law. In particular, in order to obtain a 2:1 law it is frequently necessary to shunt the rectifier networks by resistors and to include non-linear, as well as linear, impedances in series with the control path.

Another possible method of operating a system according to the present invention is to arrange for the impedance of the source at the compressor to be very high with respect to the variable impedance, whilst at the expander the impedance of the generator is very low with respect to the variable impedance; that is, in the equations above f(e) is large with respect to Z1 and Z3 is large with respect to Z5 and f(e). In this arrangement Z2 becomes equal to Z4 and Z5 is omitted.

An example of a system of this type is shown in Figures 5 and 6, which indicate the compressor and expander respectively. The necessity for the generator impedances to be made very high and very low respectively involves the use of valve amplifiers, and this arrangement therefore is not so economical as that described above.

Thus this resistance will in general be The compressor has input terminals 33 shun ed by a suitable impedance 34 when it is desired to operate from a constant impedance supply and feeding through transformer 35, the grid circuit of a high impedance valve 35, suitably of the pentode type. This valve may thus be treated as a generator of high internal impedance. The valve, which is supplied with suitable operating potentials in known manner, has an output circuit including a constant impedance 31 and a variable impedance formed by a transformer having windings 38, 39 and of which the secondary winding 39 is shunted by variable impedances 40. The voltage developed across the output impedances 31 and 33 of the valve 36 is coupled, through condenser 41 to the input of any suitable amplifying valve such as 42. The output from valve 42 is fed through transformer 43 to a balanced differential transformer 4d having a suitable balancing impedance 45. Signals appearing in the windings of transformer 43 from valve 42 are divided, by the differential transformer 44, with balance 45, between the load, which is connected across terminals 46 and the control impedance which is connected across terminals 41, in such a way that circuits 46 and 47 are mutually unaffected, according to known principles. The signals entering terminals 4'! are used after rectification in the rectifier 43 to control the imped ance of the rectifiers 43, a smoothing circuit 49 of suitable time constant being included in the circuit. The rectifier 48 may sometimes be preceded by a control amplifier which is not shown.

The corresponding expander circuit is shown in Figure 6, in which input terminals 50 are connected through a balanced differential transformer 5| to the grid circuit of a low impedance valve 54, suitably a triode. By means of the differential transformer 51 and the balance 52, the control signal is derived across terminals 53. without sensibly affecting the main transmission path. The output circuit of the triode includes a fixed resistance 55 and a variable impedance constituted by the primary winding 56 of a transformer of which the secondary winding 5! has rectifier elements 58 connected across it. The output voltage from. the valve 54 is obtained from that developed across resistance 55, and is fed, for example by transformer 58, to an amplifying valve 60 feeding, by output transformer 6| output terminals 62. In order to control the value of the impedances of the rectifiers 58 as :a function of the amplitude of the received signal a D. C. control current is obtained by rectifying that part of the input signal which appears across terminals 53 by means of rectifier 63, and applying the rectified current, after smoothing by filter 64 of suitable time constant, to the rectifiers 53. As in the case of the compressor, the rectifier 63 may sometimes be preceded by an amplifier which is not shown.

With this embodiment of the invention, in order to approximate as closely as possible to the ideal generator conditions it may be desirable to apply negative feed back to the valves associated with the non-linear networks. It is well known that when negative feed back voltage is derived from a resistance in series with the load and applied to the input, the eifective generator impedance rises, thus tending towards the condition of constant load current. Similarly, when the negative feed back voltage is derived from a potentiometer across the load resistance, the effective generator impedance is reduced, thus tending towards the condition of constant load voltage.

It is therefore clear that a suitable generator for the compressor shown in Figure 5 would be a high impedance valve such as the pentode shown with series negative feed back, and. for the expander shown in Figure 6 a low impedance valve, such as the triode, with parallel negative feed back.

It will be understood that the circuit arrangements of Figures 3, 4, 5 and 6 are given only a examples of the application of the invention and that compressor networks other than that shown may be employed. Moreover, although in the examples given the control current is obtained by rectification and smoothing of the signal current, it will be understood that the use of an alternating current for control purposes is not excluded.

Furthermore, it may be more suitable with certain types of non-linear networks to take the output voltage from across a resistance in the case of the compressor and across the nonlinear network in the case of the expander.

The use of a balanced differential transformer at the output of the compressor and/or at the input of the expander, in order that the control currents may be derived by means of rectifiers without feeding back rectified signals into the main transmission path, is an important feature of the invention.

What I claim and. desire to secure by Letters Patents is:

1. An electric wave transmission system including a device comprising an input circuit of impedance Z1, a variable impedance device included, in series with a constant impedance Z2,

in shunt to said circuit, a further device connected to said first device and having input circuit of impedance Z3 an output circuit of impedance Z4 and a variable impedance device of the same law of variation as said first variable impedance arranged, with an impedance Z5 in parallel therewith, in series in the transmission path through said second device, and means for controlling said variable impedances in accordance with the amplitude of signals passing through the system wherein the said impede-noes are substantially in accordance with the relations:

2. A system. according to claim 1, wherein the impedance Z3 is high compared with the value of the variable impedance associated therewith.

3. A system according to claim 1, wherein said input circuit includes a valve of high internal impedance.

4. A system according to claim 1, wherein the impedance Z1 of the input circuit is low in comparison with the value of the variable impedance associated therewith.

5. An arrangement according to claim 1, wherein the means for controlling the impedance of the variable attenuation device or devices includes a connection to the transmission path, said connection being made by a balancing transformer or the like whereby the effect of the control current so obtained on the transmission path is reduced.

REGINALD JOHN HALSEY. 

